Java 5 Features

IBM Global Business Services

04/10/23 Vodafone / IBM Confidential | © Copyright IBM Corporation 2010

Shashank Raj H,

10-Sept-2010

Java 5 features

KBA/COM Managed Services

2 04/10/23Vodafone / IBM Confidential | © Copyright IBM Corporation 2010

Topics

Generics

Enhanced for Loop

Autoboxing/Unboxing

Typesafe Enums

Varargs

Static Import

Metadata (Annotations)

Generics

List myIntList = new LinkedList(); // 1myIntList.add(new Integer(0)); // 2Integer x = (Integer) myIntList.iterator().next(); // 3

The idea behind generics is to enable the programmer to express his intention

List<Integer> myIntList = new LinkedList<Integer>(); // 1myIntList.add(new Integer(0)); //2Integer x = myIntList.iterator().next(); // 3 –No cast required

Compiler can check the type correctness at compile time.Improved readability and robustness.

Introduction

Cluttered

Runtime error

Small excerpt from the definitions of the interfaces List and Iterator in package

java.util:

public interface List<E> {

void add(E x);

Iterator<E> iterator();

public interface Iterator<E> {

E next();

boolean hasNext();

all occurrences of the formal type parameter (E in this case) are replaced by the actual type argument (in this case, Integer).

Generics Defining Simple Generics

List<String> ls = new ArrayList<String>(); //1

List<Object> lo = ls; //2

ILLEGAL !!

Line 2 is a compile time error.

Generics Generics and sub typing

void printCollection(Collection<Object> c) {

for (Object e : c) {

System.out.println(e);

Using wildcards

void printCollection(Collection<?> c) {

for (Object e : c) {

System.out.println(e);

Collection<?> c = new ArrayList<String>();

c.add(new Object()); // compile time error

Generics Wildcards

Object is not the supertype

for all kinds of collections.

Collection of unknown

public void drawAll(List<Shape> shapes) {

for (Shape s: shapes) {

s.draw(this);

drawAll() can only be called on lists of exactly Shape:

it cannot, for instance, be called on a List<Circle>

public void drawAll(List<? extends Shape> shapes) { ... }

Generics Bounded Wildcards

Circle Rectangle

static void fromArrayToCollection(Object[] a, Collection<?> c) {

for (Object o : a) {

c.add(o); // compile time error

We cannot just shove objects into a collection of unknown type.

static <T> void fromArrayToCollection(T[] a, Collection<T> c) {

for (T o : a) {

c.add(o); // correct

Generics Generic methods

Generics

Enhanced for Loop

Autoboxing/Unboxing

Typesafe Enums

Varargs

Static Import

Topics

FOR loop

int arr[5] = {2,4,8,16,32};

for (int i=0; i<5; i++)

System.out.println("Output: " + arr[i]);

ENHANCED FOR loop

for(int a:arr)

System.out.println("Output: "+ a); // array index not required

Enhanced for loop Introduction

Dependence on iterator

List<Integer> numbers = new ArrayList<Integer>();

numbers.add(1);numbers.add(2);numbers.add(3);

Iterator<Integer> numbersIterator = null;

for( numbersIterator = numbers.iterator() ; numbersIterator.hasNext() ; ) { System.out.println(numbersIterator.next());;

No dependence on iterator with enhanced for loop

for(Integer number : numbers) {

System.out.println(number); }

Enhanced for loop Collections with enhanced for loop

It is advantageous because It is very convenient for the programmer to iterate over a collection of elements.

It has the following drawbacks:-

-Step value cannot be incremented.

-Backward traversal is not possible.

Enhanced for loop Overview

Topics

Generics

Enhanced for Loop

Autoboxing/Unboxing

Typesafe Enums

Varargs

Static Import

Repetitive work of converting the primitive types into wrapper classes and vice - versa.

Purpose of conversion is just for some API call.

Auto-boxing and Auto-Unboxing enables the primitive types to be converted into respective wrapper objects and the other way around.

Autoboxing / Unboxing Purpose

Before Java 5

int intPrimitive = intObject.intValue();

intPrimitive++;

intObject = new Integer(intPrimitive);

Using Java 5

Integer intObject = new Integer(10);

intObject++;

Autoboxing / Unboxing Introduction

Boolean isValid = false; // Boxing

Short shortObject = 200; // Boxing

if(shortObject<20){ // unboxing

ArrayList<Integer> list = new ArrayList<Integer>();

for(int i = 0; i < 10; i++){

list.add(i); // Boxing

Autoboxing / Unboxing Example

Topics

Generics

Enhanced for Loop

Autoboxing/Unboxing

Typesafe Enums

Varargs

Static Import

public static final int COLOR_RED= 0;

public static final int COLOR_BLUE = 1;

public static final int COLOR_GREEN = 2;

This pattern has many problems, such as:

Not typesafe

No namespace

Printed values are uninformative.

Typesafe enums Previous approach

public enum Color {

RED, BLUE, YELLOW, GREEN, ORANGE, PURPLE }

The enumerated types RED, BLUE, YELLOW etc. are of type Color.

Enums are full-fledged java classes and can have arbitrary methods and fields.

Enums can implement interfaces.

Enums can have constructors and they take arguments in the declaration as shown in the next slide.

Typesafe enums Example

Java.lang.Enum

public enum Color {

RED(625, 740),

ORANGE(590, 625),

YELLOW(565, 590)

//Electro-magnetic Spectrum wavelength in nm

int startWavelength; int endWavelength;

Color(start, end) {

this.startWavelength = start; this.endWavelength = end; }

public int getStartWavelength() {

return startWavelength; }

public int getEndWavelength() {

return endWavelength; }

public static void main(String[] args) {

System.out.println("Red color's wavelength range, " + RED.getStartWavelength()+" ~ "+RED.getEndWavelength()); } }

Data members

Constructors

Methods

If the implementation of methods vary for each Constant, then you can implement the methods specific to a constant.

public enum Color {

RED { public Color complimentary() { return GREEN; }},

BLUE { public Color complimentary() { return ORANGE; }},

YELLOW { public Color complimentary() { return PURPLE; }},

public abstract Color complimentary(); }

Topics

Generics

Enhanced for Loop

Autoboxing/Unboxing

Typesafe Enums

Varargs

Static Import

Int add (int a,int b,intc c);

Int add (int a,int b,int c,int d);

Int add (int a,int b,int c,int d,int e);

Can be replaced by a single method declaration which had VAR ARGS

Int add (int… numbers){

for ( int a : numbers)

sum+=a;

return sum;

Variable arguments Example

Legal:

void doStuff(int... x) { } // expects from 0 to many ints

// as parameters

void doStuff2(char c, int... x) { } // expects first a char,

// then 0 to many ints

void doStuff3(Animal... animal) { } // 0 to many Animals

Illegal:

void doStuff4(int x...) { } // bad syntax

void doStuff5(int... x, char... y) { } // too many var-args

void doStuff6(String... s, byte b) { } // var-arg must be last

Variable arguments Examples

Generics

Enhanced for Loop

Autoboxing/Unboxing

Typesafe Enums

Varargs

Static Import

Topics

Consider the java example:

Import java.lang.math.*;

double r = Math.cos ( Math.PI * theta );

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